Automatic frequency control system



Sept. 12, 1944. c y s 2,357,984

AUTOMATIC FREQUENCY CONTROL SYSTEM Filed May. 3, 1955 4 Sheets-Sheet l INVENTOR HARLE TRAVIS ATTO R N EY p 1944. c. TRAVIS AUTOMATIC FREQUENCY CONTROL SYSTEM Filed may 5, 1955 4 Sheets-Sheet 2 mmumcr INVENTOR CHARLE TRAVIS ATT'ORNEY p 1944 c. TRAVIS 2,357,984

AUTOMATIC FREQUENCY CONTROL SYSTEM Filed May 5, 1935 4 Sheets-Sheet 3' AFC INVENTOR.

CHARLES TRAVI I ATTORNEY Patented Sept. 12, 1944 AUTOMATIC FREQUENCY CONTROL SYSTEM Charles Travis, Philadelphia, Pa., assignor to Radio Corporation of America, a corporation of Delaware Application May 3, 1935, Serial No. 19,563 Renewed April 16, 1937 Claims.

lished have in general been attained. However,

additionel investigation and experimentation has resulted in the development of further improvements in automatic frequency control systems.

The automatic. frequency control systems to be explained in detail in a later portion of this specification involve in general two distinct units. One of these units is a frequency discriminator, or frequency sensitive detector, that generates a bias varying with changes of the intermediate frequency signal carrier. control that is acted upon by the aforementioned bias, and whose function it is to vary the local oscillator frequency in a desired sense. The two units are so coordinated that if the intermediate frequency carrier tends to shift from its midband frequency value, the oscillator frequency changes sufficiently to restore proper alignment.

The automatic frequency control systems which comprise the subject matter of the present application are not only useful in facilitating the manual tuning operation of a superheterodyne receiver, but are also highly useful in maintaining the local oscillator frequency at a desired carrier setting for a long period of time, and throughout operation of the receiver during this period.

In addition to its aid and benefit in connection with the manual tuning of superheterodyne receivers, the presently disclosed frequency con- The other unit is a trol systems can be used with benefit in connection with the automatic tuning of radio receivers of the superheterodyne type. In such instances the frequency control network functions as a Vernier device to accurately tune the oscillator circuit after the automatic tuning mechanism has substantially adjusted the tuning device to its desired station position.

Accordingly, it may be stated that it is one of the primary objects of the present invention to provide improved automatic frequency control networks which are not only capable of usage in such a manner as to secure the aforementioned results, but are also constructed so as to function in a positive and reliable'manner.

"Another important object of the present invention is to provide automatic frequency control networks based on an operating principle which is substantially different from that utilized in connection with the circuits disclosed in my aforementioned patent. The present operation is based upon the fact that when the plate of the frequency control tube is coupled into the high potential Side of the resonant circuit to be adjusted in tuning, and the grid of the control tube is excited by a voltage out of phase with that appearing across the resonant circuit, then the plate current in the control tube is similarly out of phase with the resonant circuit'voltage, and the control tube presents a reactive eifect across the resonant circuit; the magnitude of this effective reactance'varies according to the frequency control bias impressed upon the controlgrid of the control tube by the frequency discriminator network.

* Another object of the present invention is to provide an automatic frequency control system for asuperheterodyne receiver, the system comprising a frequency discriminator network functioning'to provide a direct current bias which varies in magnitude with the shifting frequency of the intermediate frequency carrier; 2. reactance control tube being used in operative association with the local oscillator tank circuit in such a manner that the plate current of the control tube is out of phase with the tank circuit voltage whereby the control tube simulates a predetermined reactance across the tank circuit, the magnitude of the control reactance varying ac,- cording to the discriminator bias which is impressed upon the control grid of the control tube.

Another object of the present invention may be said to reside in the provision of a tunable oscillation circuit which is capable of being automatically tuned to different frequencies in a predetermined frequency range, there being an automatic frequency control system operatively associated with the oscillation circuit in such a manner that the oscillation circuit is accurately and automatically tuned to resonance with a desired signal carrier when the tuning means of the oscillation circuit is adjusted to a predetermined frequency, there being provided additional means for rendering the frequency control operative as soon as the oscillation circuit has been tuned to approximate resonance with said predetermined frequency.

Still other objects of the invention are to improve generally the simplicity and efficiency of au omatic frequency control systems for radio particularly characterized by their reliability and accuracy in operation.

The novel'features which I believe to be characteristic of my invention are set forth with particularity in'the appended claims. The invention itself however, both as to its organization and method of operation, will best be understood by reference to the following description taken in connection with the drawings in which I have indicated diagrammatically several circuit organizations whereby my invention may-be carried into effect.

In the drawings- Fig. 1 diagrammatically shows a superhetero-v dyne receiving system embodying one form of the present, invention,

invention applied to an automatic tuning device I for a radio receiver.

Referring now to'the accompanying drawings, wherein like reference characters in the different figures designate similar circuit ele- The high potential side of the tank circuit is connected to the control grid of tube I2 through a blocking condenser I5, the usual grid return resistor I6 being connected to ground from the grid side of condenser I5.

The cathode of oscillator tube I2 is grounded, and the plate thereof is regeneratively coupled to the tank circuit by means of a coil I4 which is magnetically coupled to coil I4. The positive potential required for the plate of oscillator tube I2 is fed to the plate through the feedback coil I4, and the low alternating potential side of the tank circuit is connected to a source of positive 1 to be described in further detail at a later point.

ments, the receiving system shown in Fig. l is of the superheterodyne type, and is shown as embodying an automatic frequency control network whose general organization ls similar to that disclosed, and claimed in my aforesaid patent.

The receiving system embodies generally a grounded antenna circuit A which feeds a radio frequency amplifier I having a tunable input which'is resonated to the desired signal frequency. The output energy of the amplifier I is'impressed' upon the tunable input circuit '3 of a first detector tube 2, the latter being shown by way of illustration as of the 6A1 type. The

' plate circuit of the tube 2 includes a tuned circuit 4 which is resonated to the operating intermediate frequency of the system, and the latter may be chosen to have a value, for example of the order-of 460 kilocycles ('k. c.) The intermediate frequency energy produced in the 'circuit t may then be amplified in one or more stages of intermediate frequency amplification,

and by way of example two such cascaded amplifier stages 5 and 6 are shown The numeral 1 denotes the tuned input circuit of the amplifier 5, whereas the numeral 8 denotes the tuned output circuit of the. amplifier.

The intermediate frequency amplifier 6 is provided with a tuned input circuit 9 and a tuned R are connected in shunt with the condenser I3.

lator input circuit II is maintained substantially The locally produced oscillations are impressed upon the grid I1, which is the nearest to the cathode of tube 2. The grid I I is connected to the high alternating potential sidev of the oscillator tank circuit through a blocking condenser I8, the grid side of condenser I8 being connected to the cathodeside of the grid bias network 20 by a resistor I9.

It will be observed that a dotted line denotes the mechanical uni-control device usually employed for operating the rotors of the variable tuning condensers of the signal and local oscillator circuits. It is to be clearly understood that the dotted line signifies the mechanical coupling between the rotors of condensers I3 and 3', and also with the rotors ofthe variable condenser usually employed in the input of amplifier I. The frequency changing function in tube 2 is accomplished by means of electronic coupling. and this action is so well known to those skilled in the art at the present time that a mere reference thereto is believed sufficient;

The signal carrier amplitude at the demoduconstant over a wide range of carrier amplitude variation at the signal collector A by means of an automatic volume control arrangement, the

latter being denoted as AVC in Fig. 1. The varying negative bias for securing the automatic amplification regulation is derived from the diode network operatively associated with the driver tube 2I. The latter tube may be of the '78 or 6D6 type, and has its control'grid coupled, for,

the impression thereon of intermediate frequency currents, ,to the high alternating potential side of the tuned output circuit 8 of amplifier 5. The signal path connected to the control grid of tube 2| includes a blocking condenser 22, the

control grid being connected to ground through a grid return resistor 23. r

The plate of tube 2I is connected to a source of positive potential through two paths; one of these paths includes the coil P1, while the other path includes the coil P2. Each of coils P1 and P2 is shunted by a condenser 24. The AVC diode network comprises the auxiliary anode 25 which is disposed adjacent a portion of the cathode of tube. Intermediate frequency energy is impressed upon the diode anode 25 through the blocking condenser 21 which is connected between the plate of tube 2| and the diode anode 2 5. The diode anode 25 has connected in circuit a direct current voltage across it when the intermediate frequency carrier amplitude attains a magnitude above a predetermined intensity level.

The anode side of resistor 28 is connected to the gain control electrodes of the controlled signal transmission tubes. In the present case the signal grids may act as the gain control electrodes, and the AVG lead is to be understood as being connected to the various grid circuits of the controlled transmission tubes. The IAVC lead is connected to the anode side of resistor 28 through a resistor 29 which acts to suppress pulsating component of rectified intermediate frequency energy; the condenser 30 connected to ground cooperating with resistor 29 to provide the well known time constant network for the AVG arrangement.

Those skilled in the art will recognize that the system described to this point is in general of a well known type. Assuming that the signal range is in the present broadcast band and covers a range of frequencies from 550 to 1500 k. c., and the oscillator frequency i greater than the signal frequency, then the local oscillator frequency range will be from 1010 k. c. to 1960 k. c. if the operating intermediate frequency is to be 460 k. c. As the intermediate frequency carrier amplitude at circuit 8 increases the negative bias applied to the control grids of the regulated tubes by the AVG network increases thereby reducing the amplification of each tube, and in this way the signal amplitude at the demodulator input circuit I I is maintained substantially uniform regardless of the signal amplitude variation at collector A.

The frequency discriminator network comprises the double diode tube 3|, and the latter may be, by way of specific illustration of the 85 type with the triode section thereof unused. The anodes of tube 3|, these anodes being designated by numerals 32 and 33, are connected to opposite sides of a resistor which develops the automatic frequency control (AFC) bias. Thi resistor has the cathode of tube 3| connected to the mid-point thereof, and therefore a single resistor may be employed or a pair of resistors of equal magnitude may be used instead. In any event the numeral 34 designates the resistor section between the cathode of tube 3| and the anode 32.

The resistor section 34' designates the load between the cathode of tube 3| and the diode anode 33. Each of the resistor sections 34 and 34 is shunted by a bypass condenser, and the anode side of resistor 34 is grounded. The secondary coil S1, which is magnetically coupled to primary P1, is connected between anode 32 and resistor section 34. The secondary coil S2 is magnetically coupled to primary P2, and the secondary is connectedbetween the anode 33 and the grounded side of resistor section 34. A condenser 35 is connected in shunt with coil S1 and tune the latter to a frequency located on one side of the operating intermediate frequency value, whereas condenser 36 is in shunt with coil S2 and tune the latter to a frequency located on the other side of the operating intermediate frequency.

The frequency control tube is shown by way of illustration as being of the pentode type, and its plate is connected by lead 31 to the high alternating potential side of the oscillator tank circult. The cathode of control tube 38 is grounded through a grid bias network 39, and the control grid of tube 38 is connected to the anode side of resistor 34 through a path which includes the grid return resistor 40 and a lead 4|. The lead 4| is designated by the letters AFC and is to be understood as denoting the automatic frequency control bias path. The lead 4| is adapted to be connected to ground by a switch 42, and it will be understood that the automatic frequency control bias path for tube 38 is operative only when switch 42 is open. When the switch 42 is closed, then the AFC path is short circuited to ground and is inoperative. The control grid of tube 38 is also connected to the upper end of resistor R in the tank circuit of the local oscillator through a blocking condenser 43, a bypass condenser 44 being connected between the low alternating potential side of the tank circuit and ground.

In considering the operation of the AFC system of the receivershown in Fig. 1 it is first pointed outthat a high degree of selectivity in a supernal frequencies have been increased by that amount without changing the intermediate fre-' quency band width, i. e., at 20 megacyles (mc.) the nominal 10 k. 0. intermediate frequency band is only 0.05% of the base frequency. To meet this increase in selectivity in present day receivers manualtuning means have been improved by the employment of more smoothly working speed reducing mechanical movements to operate the variable tuning condenser.

Nevertheless the maintenance of proper tuning after the station signal ha once been correctly brought in, is a problem that requires a reliable and accurate solution. Local oscillator drift, if not corrected by more or less frequent manual readjustment, is capable of mistuning th signal by many channels in the course of a few hours run. In the broadcast band conditions are quite as serious if quality of reproduction is a consideration; The averagebroadcast set user does not tune the receiver well enough to obtain the best quality it is capable of giving; this is due not only to negligence, but to a great extent because of the lack of necessary skill, and in the latter case the mechanical design of the set is a contributing factor. These consideration show the need for supplementing the accuracy of manual or automatic tuning by an automatic frequency control device.

It will therefore be seen that it is the essential object of the frequency discriminator and frequency control tube to cooperate to adjust the oscillator frequency so as to make it appear as if the signal carrier frequency has been adjusted precisely to the center of its I. F. (intermediate frequency) band, and to anchor the receiver there in spiteof small original maladjustments of tuning, or other causes, that subsequently arise from thermal changes and the like. It will now be seen that the frequency discriminator network generates a bias, applied through lead 4|, which varies with changes of the predetermined I. F.

'value; the control tube 38 has its gain varied by the .AFC bias, and the control tube functions to vary the local oscillator tank circuit in frequency. The discriminator and control units are so interrelated that if the I. F. carrier frequency tends to move away from the operating I. F. mid-band position, then the oscillator frequency changes sufficiently to restore the proper alignment. Such units and their construction for performing these of an inductance. V ance may be considered to consistof two parts,

functions are generally disclosed in my aforementioned patent,

The two similar I. F. transformers, between tube 2| and tube 3|, have their primaries P1 and P2 connected in parallel in the plate circuit of the driver tube 2|. This composite primary is tuned 'to the mid-band frequency of the I. F. band; in

'aid in de-coupling the secondaries from each other and so causes them to act like isolated single tuned circuits.

The secondaries S1 and S2 are each connected into one of the plates of the double diode tube 3|,

and the cathode of this tube is floating for direct current voltage. One of the diodes returns to ground, and the AFC bias output is taken from the other diode return. The output of the discriminator is the algebraic difference between the rectified outputs of the two diodes. If the I. F. carrier frequency is off center frequency or the operating I. F. value towards the resonance of coil S1, then the diode including anode 32 will produce the greater rectified voltage output of the two diodes, and the generated AFC bias will be negative with respect to ground. Conversely, if the I. F. carrier frequency is off in the other direction the reverse will be true. That is to say, the

'AFC bias will be positive in polarity. When the carrier frequency is exactly aligned thebias on the grid of tube 38 will equal the initial bias value thereof since the AFC bias will then be zero.

It will be noted that the AVG diode is driven from the driver tube which feeds the discriminator network. This mode of generating AVC bias is employed, in preference to deriving AVC input energy from the circuit 1 l because there is thus obtained a higher carrier input into the AVG rectifier. A considerable voltage delay may be used on' the AVG diode. Another advantage of taking off the AVG bias in this manner is that the 5 signal modulation at the second detector is not distorted by the demodulation produced by the AVG diode.

' g If the AFC driver tube also drives the AVG rectifier its output is automatically held nearly constant no matter where it is connected. 'Therefore, the grid of the driver tube'may be connected at the primary or secondary of the transformer preceding the last I. F. amplifier tube. This has the merit of giving a broader channel for the coincide, The control circuit acts to vary the oscillator frequency and this variation may be thought of as if due to a variation in capacity, or

The total tank circuit reactreactance reflected across coil 14.

viz., that part physically present as in the variable tuning condenser, or the tuning coil, and. that part reflected by the control tube circuit. The latter is a function of the I. F. carrier frequency, and its magnitude varies with the carrier frequency shift from I. F. mid-band frequency.

In my aforementioned patent the control tube has been shown as reflecting capacity into the tank circuit, or the plate resistance of the control tube has been connected in series with a reactive element in the tank circuit. Additional investigation and experimentation have revealed fre quency control circuits which make use of a different principle. It will be noted that the plate of control tube 38 is coupled into the high potential side of the tank circuit, and that the grid of the control tube is excited by an alternating voltage which is out of phase with that appearing across the tank circuit. The plate circuit in-the control tube is then likewise out of phase with the tank voltage, and accordingly the plate to cathode impedance of control tube 38 looks like a reactance to the tank circuit.

The sign of the reactance presented to the tank circuit by the control tube depends upon the nature of the impedance across which is developed the alternating voltage to be fed. to the input electrodes of the control tube. The magnitude of the reflected reactance depends upon the value of the AFC bias impressed upon the control grid of control tube 38, as this varies the mutual conductance, and proportionately the magnitude of the plate current, of the control tube. In Fig. 1 there is shown one method of obtaining the out-of-phase excitation for the grid of the control tube, and it will be noted that this is accomplished by taking the alternating voltage across resistof tube 38 simulates an inductive reactance across a coil I4. As' the plate current of tube 38 is decreased the shunting effect of the reflected or simulated inductance is decreased, and. the effective inductance of the tank circuit will therefore increase. This results in a decrease of oscillator frequency. A specific example will be given to illustrate the operation of the control tube 38. Assume that the set is to be tuned to a signal of 600 k. c.; the local oscillator frequency if higher will have to be 1060 k. c. to produce an I. F. of 460 k. c. As the tuner device is adjusted from 500 to 600 k. c., the oscillator condenser I3 tunes the tank circuit from 960 to 1060 k. c. When the tank circuit is within 2 k. c., for example, of 1060 k. c. (1058 k. c.) the I. F. value is 458 k. c. Since this is the frequency of the circuit S2, rectified voltage will be developed across resistor 34'. The control grid of tube 38 will, therefore, become less negative than its initial negative bias value with respect to ground due to its connection through lead 4| and resistor 34 to the cathode side of resistor 34'. The plate current flow of tube 38 increases; this will result in an increase in the inductive Thus, the effective inductance in the tank circuit decreases; the oscillator frequency, therefore, increases.

The increase in frequency of the tank circuit will be such as to bring the local oscillation fre quency to 1060 k. 0. Then the proper I. F. value of 460 k. c. is secured, and the set-sounds tunedin to the listener. The reverse action takes place, of course, when the set is tuned away from the desired setting of 600 k. c. Within 2 k.-c. on

either side of 600 k. c. the AFC system will act to anchor the tuning of the receiver. When the receiver is tuned beyond the permissible limits of a carrier setting, the AFC action is suspended until the limit of the adjacent channel is reached. In this way the normally highly selective superheterodyne receiver is rendered easily tunable; a range of some 4 k. 0., for example, is provided within which the set may be turned and yet distortionless reproduction be secured. The same operation is secured when the local oscillator circuit constants vary due to thermal, or other, effects. The AFC acts to bring the oscillator frequency back to the desired operating value so as to produce the operating I. F.

Ehere are numerous methods of obtaining the out-of-phase grid voltage for the control tube 38. In order to make it clear to those skilled in the art how such different modes of procedure may be achieved, the following analysis of the operation of tube 38 and its circuits is furnished. It is to be clearly understood, however, that the analysis is theoretical in nature, and in no way affects the demonstrated operation of the reactance control network,

It can be shown, in general, for an oscillator network of the type illustrated in Fig. 1 (that is,

the tank circuit and the tube 38 with its circuits) that the combination of the control tube 38 and the oscillator presents to the tank circuit an effective impedance which is the negative reciprocal of the control tube impedance with respect to the resistance product of both oscillator and control tubes. For example, a capacity is the reciprocal of an inductance with respect to a resistance product; the reciprocal of a shunt combination is a series combination. I

A practical resistance value for resistor R in Fig. 1, for a broadcast local oscillator, is about 100 ohms. With the Gm (mutual conductance) of tube 38 qual to 1000 micromhos, the frequency change is about percent. This permits a variation of plus or minus 25 k. c. at 1 mc., which seems ample for the purpose. The tuned impedance of the tank circuit is unaffected by the control action. Constant oscillator amplitude with frequency change is one of the advantages of the present arrangement.

The series resistor R is not confined to the inductive leg of the tank circuit. For example, and as shown in Fig. 2, the resistor R1 is placed in the capacitive leg of circuit I3I4. In this case the control tube cathode to plate impedance will appear as a resistance-capacity arm across the tank circuit; increase in the Gm of the control tube will now lower the oscillator frequency. It is necessary, in this case, to reverse the AFC connection to the discriminator network. Lead 3| is, therefore, connected to the anode side'of resistor 34 and the anode side of resistor 34 is grounded.

Another method of securing the out-of-phase grid voltage for tube '38 is shown in Fig. 3. Here the reactive element is condenser C which is in series with the oscillator tickler coil I4. Condenser C is grounded, and choke 50 is used to feed the direct current voltage to the plate of 0s.- cillator tube I2. The current flowing in the external plate circuit of tube I2 is in phase with the tank circuit voltage. A reactive element in the plate circuit will, therefore, develop an outof-phase voltage which is impressed upon the control grid of tube 38. The cathode to plate impedance of tube 38 presents a negative inductance across the tank circuit; the net result is an in crease in effective inductance in the oscillator tank circuit. The AFC connection must, then, be reversed in polarity with respect to that shown in Fig. 1.

If the capacitive reactance C is replaced by an inductance coil then a negative capacity will be reflected across the tank circuit. 'If a negative inductance, say the mutual of a transformer, were used in place of C, it would bereflected as a positive Capacity. Any resistive element reflects as a negative resistance, or conductance, and tends to aid the feedback of the oscillator.

In Fig. 4 is shown a modification of the method of securing out-of-phase voltage; it'diifers from that of Fig. 3 in that the out-of-phase voltage is taken from across the tickler coil I4. The plate to cathode impedance of the control tube, in this case, presents a negative capacity across the tank circuit. It is not necessary to change the AFC connection to the discriminator network from that shown in Fig. 1.

It may be that the reflected reactance from the tank circuit will give rise to an 'in-phas'e component across coil I4. This may be eliminated by using the arrangement shown in Figs. '5 and 6. In Fig. 5 the coil L1 is coupled to tickler I4, it is not coupled to tank coil I4. The out-of-phase voltage across with is impressed upon the con trol grid of tube 38 through coil L2; thelatte'r coil is coupled to coil L1, The plate to cathode impedance of control tube 38pres'ents a negative capacity across the tank circuit; the AFC connection is the same as that in Fig. 4. In Figfi is shown the effect of reversing the end connections of coil L2. The plate to cathode 1m.- pedance of tube 38 now presents a. positive capacity across the tank circuit. The AFC connection must be reversed in polarity as shown. It is, further, pointed out that the circuit of Fig. 3, where.- in the out-of-phase voltage is derived across C, can be used to avoid the in-phase componfint of Fig. 4.

Actual experience with automatic frequency control circuits of the type referred to herein has shown that the out-of-phase voltage for the control tube 38 is best secured fromacros's a condenser. This causes the plate to cathode 11mpedance of the control tube to'produce'an in ductance variation, and realizes a range and sensitivity of control which are proportional to frequency over each particular tuning band. Equalbias change on the control tube will produce equal change in the tank circuit inductance; with capacity as the tuning element this will give equal percent frequency change. Under certain conditions of use the control tube maybe affected by the oscillator; a certain degree of self-maintained frequency modulation may occur.

This is effectively prevented by using the arrangement shown in Fig. 7. In this case the con denser C1, which supplies the out-of-phase volt..- age to control tube 38, is in the plate-circuit of the electron coupled oscillator tube I2. 1 The oscillator tube I2 is shown of the 6A! type I by way .of specific illustration, and uses the 'normal electrode voltages shown. The fourth grid of the oscillator is not used, but is bypassed to the cathode by a condenser and given a fixed bias of 3 volts. The condenser C1 coupling to the control grid .of tube 38 is placed in the plate circuit of the 6A7 tube with parallelplate voltage feed around it through a radio frequency chok coil 60. A resistor 6I in series with tickler coil I4 in the plate voltage line to the oscillator tube serves to reduce the voltage to the circuit l3-l4 through the condenser 62.

" matic tuner.

proper value for the oscillator anode grid I8. I It will be noted that the first grid functions as the oscillator grid, and is coupled to tank The tickler coil [4' regeneratively couples the second or anode grid to the tank circuit. Local oscillationsfor the grid ll of the mixer tube 2 (Fig. 1) are tapped off from any desired point on coil 14. Similarlythe plate of control tube 38 may be adjustably connected to any desired point on coil l4. These adjustable connections to coil I4 maybe used in connection with any of the other arrangements if desired. The alternating current component of the plate current i of the tube flows through condenser C1 and gives the necessary out-of-phase alternating voltage 'for the control grid of tube 38. Since the alternating current component of plate current flows out of a; high plate resistance, and condenser Ci is connected right across the plate, the circuit is not complicated.

The condenser C1 and choke 60 may be replaced by the primary winding of an I. F. transformer. In the broadcast band this combination of condenser and choke may resonate J'ust'under the lowest frequency of the band so that the effective value of C1 for oscillator frequency is less I at the low'end of the band resulting in more uniform AFC sensitivity.

"In Fig. 8 there is shown a variation of the arrangement of Fig. 7; in this case the oscillator is shown as including a pentode tube of the same type as tube 38. The oscillator'tube II has its cathode magnetically coupled, as at M1, to the tunable tank circuit 12. The plate of tube 38 is connected by lead 13 to the grid side of tank circuit 12. The plate of oscillator H is fed with positive voltage through coil ID; the condenser C2 feeds the out-of-phase' alternating voltage to' thein'put grid of tube 38 through blocking condenser 43.

, In Fig. 9 there is shown, in schematic manner,

the application of the invention to an automatic local oscillator circuit, an intermediate frequency control switch is depressed. The condenser I3 is that in the oscillator tank circuit. The AFC system is to be understood as being that of Fig. 1. In fact, the entire system may be that of Fig. 1, or any of the AFC networks of the other figures. The switch 42 is the switch of Fig. 1 which is capable of "short circuiting' the AFC line when closed. The movable element 42' of switch 42 is coupled to the'armature 99 of a solenoid; an insulation spacer 9| is provided between 42 and 90. V

The'armature 90 carries a'winding 92 which is connected to the tuning motor armature current supply. A fixed field current winding 93 causes the element 42 to contact the other element of switch 42 Whenever the tuning motor is operating and varying the condensers. Whenthe tuning motor ceases to run, the armature 99' is pulled back by spring 94; this opens switch 42 V and the AFC is operative.

Thus, in Fig. 9 there is: shown a system capable of vernier tuning after rough'tuning by an auto- The AFC here operates to bring the tank circuit into exact tune after the tuning motor has stopped running. As long as the motor is turning the condenser rotors, theswitch 42 g is closed. This renders the AFC inoperative because the switch 42 short circuits the AFC sys tem. The flexibility of the present invention in so far as variety of uses is concerned will now be seen.

In any of the systems shown in Figs. 1 to 8 inclusive, the control tube 38 can be a sharp cut-off R. F. (radio frequency) pentode such as the 6C6 type tube. The coupling to the mixer tube from the local oscillator is not limited to that shown in Fig. 1. In the arrangement of Fig. 'I, for example, the grid fourth from the cathode may have signals impressed thereon. This would permit the 6A7 tube shown there to act as a combined oscillator-first detector.

The control tube 38 should be operated at conservative voltages so that its life will exceed that of the other tubes. Further, a push-pull variant of the single side circuit may be used to prolong the life of the control .tube. The AFC action should be slower than the AVG. This is to prevent a signal from being brought into the center of the band, where gain is high, more rapidly than the gain can be reduced by 'AVC action; otherwise systems for carrying my invention into effect, it

will be apparent to one skilled in the art that my invention is by no means limited to the particular organizations shown and described, but that many modifications may be made without departing from the scope of my invention, as set forth in the appended claims. 7

What I claim is:

1. A superheterodyne receiver comprising a frequency changer network including a tunable network, means for demodulating the intermediate frequency energy, a discriminator network, connected to the intermediate network, to have intermediate frequency energy impressed thereon, constructed and arranged to produce a dinected to the oscillator circuit, a phase shifter in the oscillator circuit, said phase shifter including a resistor element across which is developed an alternating voltage substantially in quadrature with the output electrode alternating potential of said electron discharge tube, said resistor element cooperating with one of the reactive tuning elements of said tunable oscillator circuit to provide said phase shifter, means for impressing said quadrature voltage upon the input electrodes of said tube, and means'for impressing the direct current voltage produced by said discriminator upon an input electrode of said tube whereby the frequency of said local oscillator circuit may be adjusted in a predetermined direction.

2. A superheterodyne receiver comprising a frequency changer network including a tunable local oscillator circuit, an intermediate frequency network, means for demodulating theintermes diate frequency energy, a discriminator network; 7'

connected to the intermediate network to have intermediat frequency energy impressed there;

energy, an electron discharge tube including at least an output electrode and a pair of input electrodes and having its output electrode connected to the oscillator circuit, a phase shifter in the oscillator circuit, said phase shifter consisting of a reactive component of the tunable circuit and a resistor element, said resistor element being chosen to develop thereacross an alternating voltage substantially in quadrature with the output electrode alternating potential of said electron discharge tube, means for impressing said quadrature voltage upon the input electrodes of said tube, and means for impressing the direct current voltage produced by said discriminator upon an input electrode of said tube whereby the frequency of said local oscillator circuit may be adjusted in a predetermined direction.

3. In combination with a resonant circuit including a condenser and coil, an electron discharge tube provided with at least an anode and a pair of input electrodes and having its anode connected to a point of relatively high alternating potential on said resonant circuit, a phase shifter in said resonant circuit, said phase shifter consisting of a resistor element in series with said resonant circuit condenser, connections from the input electrodes of said tube to said resistor for impressing the alternating voltage developed across said resistor upon the input electrodes of said tube, said voltage being substantially in quadrature with the alternating potential at the anode of said tube, and means for varying the space current flow of said tube,

4. In combination with a resonant circuit including a condenser and coil, and said condenser being variable to tune the circuit over a Wide range of frequencies, an electron discharge tube provided with at least an anode and a pair of input electrodes and having its anode connected to a point of relatively high alternating potential on said resonant circuit, an impedance connected in said resonant circuit, connections from the input electrodes of said tube to said impedance for impressing the alternating voltage de- Veloped across said impedance upon the input electrodes of said tube, said voltage being sub stantially in quadrature with the alternating potential at the anode of said tube, means for varying the space current flow of said tube in response to variations in frequency of said resonant circuit Within predetermined limits of adjustment settings of said resonant circuit variable condenser, and said impedance being a resistance in series with the coil of said resonant circuit.

5. In combination with a resonant circuit including a condenser and coil, and said condenser being variable to tune the circuit over a wide range of frequencies, an electron discharge tube v provided with at least an anode and a pair of input electrodes and having its anode connected to a point of relatively high alternating potential on said resonant circuit, an impedance connect- 6. In combination with a local oscillator network of a superheterodyne receiver, said oscillator network including an electron discharge tube having a cathode, a plate and at least two auxiliary electrodes, a tunable tank circuit connected to one of the auxiliary electrodes, and means for regeneratively coupling the other auxiliary electrode to said tank circuit, a frequency control tube provided with at least a plate and a pair of input electrodes, the control tube having its plate connected to a point of relatively high radio frequency potential on said tank circuit, a condenser in the plate circuit of said oscillator tube across which is developed an alternating voltage substantially 90 out of phase with the alternating potential at the plate of said control tube, connections for impressing said out of phase voltage upon th input electrodes of said control tube, and ,means for varying the plate current flow of said control tube.

7. In combination with an electron discharge tube having input and output circuits reactively coupled to produce oscillations, at least one of the coupled circuits being resonant to a desired oscillation frequency, means for varying the frequency of the resonant circuit comprising a tube having at least an input electrode and an output electrode, means feeding alternating current from the second tube output electrode to a point of relatively high potential of said resonant circuit, means including a condenser electroncoupled to said resonant circuit, for impressing upon said input electrode alternating voltage derived from the oscillation tube which is in phase quadrature With the alternating potential at said point, and means for controlling the gain of said second tube.

8. In combination with an electron discharge tube having input and output circuits reactively coupled to produceoscillations, at least one of the coupled circuits being resonant to a desired oscillation frequency, means for controlling the frequency of the resonant circuit comprising a tube having at least an input electrode and an output electrode, means feeding alternating current from the second tube output electrode to a point of relatively high potential of said resonant circuit, means, including a reactance electronically coupled With said resonant circuit, for impressing upon said input electrode alternating voltage derived from the oscillation tube which is in phase quadrature with the alternating potential at the output electrode of the second tube, and means forcontrolling the gain of said second tube.

9. In combination with an electron discharge tube having input and output circuits reactively coupled to produce oscillations, at least'one of the coupled circuits being resonant to a desired oscillation frequency, means for adjusting the frequency of the resonant circuit comprising a tube having at least an input electrode and an output electrode, means connecting the second tube output electrode to a point of relatively 7 high potential of said resonant circuit, means for impressing upon said input electrode alter nating voltage derived from the oscillation tube which is in phase quadrature with the alternating potential at the output electrode of the second tube, and means for controlling the gain of said second tube, said impressing means including a reactance in the space current path of the oscillation tube and electron coupled to said resonant circuit.

10. In combination with an oscillatory system provided with an oscillator tube and a resonant tank circuit provided with a tuning reactance, an electron discharge device provided with a cathode and at least a positive output'electrode and an input electrode, means connecting the cathode to output electrode impedance of said electron device across said tank circuit, a phase shifter for said tank circuit comprising said tuning reactance and a resistance in series, said resistance being located between the input electrode and cathode of said electrondevice, said resistance providing thereacross an alternating voltage substantially phase-shifted relative to the tank circuit voltage, means applying said phaseshifted voltage to said input electrode thereby to cause said impedance to exercise an essentially reactive efiect on said tank circuit, and means for varying theelectron flow to said output electrode in response to a variable voltage thereby to'vary said reactive effect. a

11. In combination with an oscillatory system provided with an oscillator tube and a resonant of said electron device across said tank circuit,

a phase shifter element, means in said oscillator tube electronically coupling said element to said tank circuit, said element providing thereacross an alternating voltage phase-shifted substantially 90 degrees relative to the tank circuit voltage, 7

means applying said phase-shifted voltage to saidinput electrode thereby to cause said impedance to exercise an essentially reactive efiect on said tank circuit, and means for varying the electron flow to said output electrode in response to a variable voltage thereby-to vary said reactive effect.

12.:In combination with an oscillatory system provided with an oscillator tube and a resonant tank circuit, said tank circuit comprising at least two reactive elements of opposite sign cooperating to tune the system to a desired frequency, means for varying the frequency of the oscillations produced by said system, said means comprising an electron discharge device provided with a cathode and at least a positive output electrode and an input electrode, means connecting the cathode to output electrode impedance thereby to vary said simulated reactance.

13. In combination with an oscillatory system provided with an oscillator tube and a resonant tank circuit, said oscillator tube having a cathode and at least 'two cold electrodes forming an oscillater section, said tube including an additional output electrode electron coupled to said oscillator section, an electron discharg device provided with a cathode and at least a positive anode and an input grid means connecting the cathode to anode impedance of said electron device across said tank circuit, a phase shifter comprising a capacitance, said capacitance being located between the input grid and cathode of said electron device, said capacitance providing thereacross an alternating voltag phase-shifted relative to the tank circuit voltage, means applying said phase-shifted voltage to said input grid thereby to cause said impedance to exercise an essentially reactive effect on said tank circuit, and means for varying the electron flow to said anode in response to a variable voltage thereby to vary said reactive effect.

14. In combination with an oscillatory system provided with an oscillator tube and a resonant tank circuit, said tank circuit comprising at least two parallel-connected reactive elements of opposite sign co-operating to tune the system to a desired frequency, means for varying the frequency of the oscillations produced by said system, said means comprising an electron discharge tube provided with a cathode and at least a positive'output electrode and an input electrode, means connecting the cathode to output electrode impedance of said electron device across said tank circuit, said last means including a direct current connection'from said output electrode to the high alternating potential side of said tank circuit, a resistor electrically connected in series with one of said two reactive elements toprovide a phase shifter, said resistor being chosen to provide an alternating voltage phaseshifted substantially degrees relative to the tank circuit voltage, means applying said phaseshifted voltage to said input electrode thereby to cause said impedance to simulate a e fln in said tank circuit, said applying means comprising a direct current blocking condenser connected from the junction of saidresistor and one reactive element to said input electrode and means for varying the electron flow to said output electrode in'response to a variable voltage thereby to vary said simulated reactance.

15. In combination with an oscillatory system provided with an oscillator tube and a resonant tank circuit, means for varying the frequency of the oscillations produced by said system, said means comprising an electron discharge tube provided with a cathode and at least a positive output electrode and an input electrode, means connecting the cathode to output electrode path of said electrondischarge tube across said tank circuit, a phase shifterelement, said oscillator tube comprising a cathode, control grid, positive screen grid and positive plate, said cathode, control grid and screen grid being coupled to the tank circuit to provide said oscillations, means connecting said phase shifter element in circuit with said plate for electronically coupling said element to said tank circuit, said element providing thereacross analternating voltage phase-shifted substantially .90 degrees relative to the tank circuit voltage, a condenser applying said phase-shifted voltage to said input electrode thereby to cause said impedance to exercise an essentially reactive elfect on said tank circuit, and means for varying the electron flow to said output electrodein response to a variable voltage thereby to vary said reactive effect.

CHARLES TRAVIS. 

